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Calculating the fan capacity required for ventilation

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You are here:- home > cooling index > fan index > fan assisted cooling

Fan assisted cooling for medium to large commercial buildings

It is rare in the UK for the summer temperature outdoors to exceed 32C. However the temperature inside modern commercial buildings such as factories, workshops and warehouses is often well over 45C making them very uncomfortable to work in.

High indoor temperatures have several causes:
Solar gain – The sun heating the roof and the air immediately below it.

Machinery – Equipment inside the building generating heat.

Workers – Every occupant inside the building produces heat.
Because we live in a reasonably temperate environment, it is possible to use air from outside to effectively cool buildings in all but the hottest spells. Fans can be used to remove hot air, supply cooler outdoor air and create air movement inside industrial buildings.

Using axial fans to cool buildings

Using axial fans to cool buildings is a three-step process:
1) Use fans to remove hot air – Hot air gathers in the roof space, which in turn heats the air below, using roof extract fans to expel this air is the most effective way to reduce indoor temperatures.

Wall extractor fans should also be positioned adjacent to machines that produce heat. So that they can remove the hot air created before it has the chance to mix with and raise the temperature of the surrounding air.

2) Supply fresh cooler air from outdoors – Extracting hot air produces negative pressure within the building, causing air to rush in when doors and windows are opened. Supply fans replace the hot air with cooler ambient (outdoor) air. Supply fans can be combined with polythene ducting to improve air distribution.

Supply fans should be installed at the opposite end of the building from the extract fans to create airflow along the length of the building.

3) Install fixed or portable air circulation fans – fixed overhead or portable floor fans are used to create a comfortable breeze, to cool workers, and overcome hotspots. They should be installed to push air from the supply fans towards the extract fans.

Selecting fan size using air speed

As the speed of the air around the skin increases so does the cooling effect. However there is a fine balance to be achieved. Below 0.7 m/s (138 ft/min) workers will feel a pleasant breeze; above this papers and hair start to move. Over 1.7 m/s (335 ft/min) the breeze becomes unpleasant in all but the hottest conditions.

The air speed needed for effective cooling depends on the length of the building:

Recommended air speeds
Building length Air speed
Upto 30m (100ft.) 0.765m/s (150ft/min)
30m to 60m (200ft.) 1m/s (200ft/min)
60m to 90m (300ft.) 1.275m/s (250ft/min)
Over 90m 1.275m/s (250ft/min)

Additional overhead circulation fans should be installed every 15m (50ft.) at a height of approx 3m (10ft.) to assist airflow. In wide buildings they should be 6m (20ft) apart.

Calculating fan size

The fan size is found as follows:

Fan size (m3/sec) = Cross sectional area of the building * air speed


A factory unit measuring 4m high by 8m wide by 25m long is to be cooled using supply fans at one end and extract fans at the other. There is no heat producing machinery in the unit.
Determine the air volume required for fan assisted cooling.

The building is less than 30m long so the required air speed is 0.765 m/s

Cross sectional area = height * width = 4 * 8 = 32 m2

Air volume = CSA * air speed = 32 * 0.765 = 24.5 m3/s

Extract and supply fans with a capacity of 24.5 m3/s should be chosen.
Because the building is less than 90m long additional circulation fans are not needed.


When industrial buildings get too hot most businesses consider air conditioning, which is both expensive to install and operate. Additionally air conditioning is not suited to cooling large open spaces with high ceilings and open doors, windows, loading bays etc.

Fan assisted cooling using axial fans, is much cheaper to install and operate, and is ideal for modern industrial buildings. With careful planning the temperature can be controlled to within a few degrees of the ambient (outside) temperature creating much more pleasant working conditions.

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